The VOX Showroom - The 7120 and The Beatles.

Technical Specifications of the 7120

The VOX 7120 is a hybrid amplifier with a solid state pre amp section and a tube driven power section. The amplifier is divided into two channels with two inputs per channel. The vibrato channel contains a master volume control plus treble, mid-range, and bass tone controls. In addition, it has vibrato effects. This vibrato effects includes two controls. One for strength of the signal and the second one for the timing of the vibrato channel. In addition, it has a distortion effect. The vibrato and distortion are both activated by a foot switch. Finally, the vibrato channel has an upper mid-range boost. This is activated by a control switch on the panel. The normal channel has a master volume control and controls for treble, mid-range and bass tone. It also has controls for reverb. Finally, it contains a top boost feature on the control panel. All the effects features (vibrato, distortion and reverb), except boost, are activated by a three pin foot switch.
The unique feature about the amplifier is that the design is extremely complex for a 1966 guitar amplifier. Specifically, the power unit of the amplifier utilizes four KT-88 tubes with two driver tubes. One EL84 and one ECL86. According to the VOX catalogue the minimum power output the amplifier can produce is 120 watts RMS. Given this set of power tubes approximately 180 watts output is well within the capacity of this amplifier. The amplifier has an ultra-linear output stage powered by albion transformers. This set up is designed to accurately reproduce and amplify the signal submitted to it by the pre-amp section of the amplifier by means of negative feed back loops. Therefore, all musical notes are faithfully reproduced at the same volume level and same tonal quality in much the same way that a very expensive high fidelity system would reproduce the signal from a record. Therefore, the signal reproduced is extremely clean and powerful under this design. In addition, each KT-88 tube has its own separate bias control which allows for a fine tuning of bias for each tube to allow for a maximum output with a minimum of distortion. The pre-amp section of the amplifier is based upon signals generated by solid-state transistors. The type of transistors used at the time were transitional. Prior transistors used in JMI amplifiers were made of germanium. For example, Germanium transistors were used in the VOX T-60 base model. Later, FET transistors were used in the American VOX solid state models such as the Beatle manufactured by Thomas Organ. The transistors used in the 7120 model were silicon transistors designated as BC-107 and BC-109 transistors. The vibrato for the amplifier has an AC-128 transistor. The reverb for the amplifier has an AC-127 transistor. The reverb is powered by a single spring suspended unit.
The 7120's as used by the Beatles differed in several respects from the production 7120's. The differences appeared to be cosmetic. First of all, the chassis of the Beatles' 7120's appeared to be smaller. They had dimensions of 22-1/2" w x 8-1/2" h x 11-1/4" d. As compared to production models which had dimensions of 24-1/4" w x 9-1/4" h x 11-1/4" d. Their models were approximately the same size as a VOX 730 and VOX 715. In addition, the handle configuration of the Beatles 7120 placed one handle in the top center of the amplifier head whereas the production models had two handles on either side of the model amplifier head. Third, the Beatles' 7120's included a removable back cover. It was removed via four screws located in the corners at the back of the amplifier. This allowed easier access to service the tube of the unit. Fourth, the Beatles' housing for the KT88 tubes was inclined at a slight angle which appears to be approximately 25 - 30 degrees so that the tubes are not sitting directly upright but rather at an angle and the flooring for the tubes is ventilated along regular intervals. This feature was probably designed to assist with ventilation of heat from the tubes. Finally, the vents of the Beatles' 7120's are located more clearly in the corners of the amplifier than in the production models which are more closely centered towards the middle of the amplifier.
The other distinguishing features with respect to the Beatles use of the VOX 7120 is the cabinet used by them. At the time of this writing, only one original era cabinet is known to exist and this is a single cabinet with two Celestion 12" and two 10" Celestion Alnico speakers. This cabinet is clearly shown on page 63 of The VOX Story by David Peterson and Dick Denny. The Beatles cabinet was considerably larger than this cabinet and was, in all probability, also an open backed cabinet with at least four 12" Celestion high powered speakers and two middax horns. Note, there is also speculation that the Beatles unit may have been powered by four 12" speakers and two 10" Alnico speakers.